1. Field of the Invention
The present invention relates generally to CMOS image sensors, and more particularly, to analog-to-digital conversion in the CMOS image sensor with correlated double sampling (CDS) using a switched capacitor array for generating ramp voltages.
2. Background of the Invention
An image sensor captures an image using an array of photodiodes that react to light. Since the brightness and wavelengths of light emitted from different portions of an object are different, each of the pixels in the array of photodiodes generates a corresponding electrical value that is converted by the image sensor into a form for further signal processing.
A charge coupled device (CCD) is common in some conventional image sensors. However, with the rapid development of CMOS (complementary metal oxide semiconductor) technology, an image sensor now includes CMOS transistors, being referred to as a CMOS image sensor. A CMOS image sensor includes an analog-to-digital converter (ADC) while an ADC is implemented by a separate integrated circuit in the CCD image sensor. In addition, the CMOS image sensor operates at lower voltage, consumes lower power, and is readily integrated into a standard CMOS process, in contrast to the CCD image sensor. Therefore, the CMOS image sensor is already used in many fields and is expected to replace the CCD image sensor in many other fields.
FIG. 1 is a block diagram of a conventional CMOS image sensor 10. Referring to FIG. 1, the CMOS image sensor 10 includes a pixel array 20, a control circuit 30, a row driver 40, an ADC block 50, and a column driver 60.
The pixel array 20 includes a plurality of pixels (not shown) arranged in a matrix for sensing an image. The control circuit 30 generates a plurality of control signals for controlling the row driver 40 and the column driver 60. The row driver 40 selects a particular row in the pixel array 20 in response to such control signals. The ADC block 50 receives pixel signals from pixels of the selected particular row and converts the pixel signals into digital signals. The column driver 60 selects a digital signal for a particular column in response to the control signals from the control circuit 30.
FIG. 2 is a circuit diagram of a conventional 4-transistor unit pixel 200 in the pixel array 20 of FIG. 1. Referring to FIG. 2, the unit pixel 200 includes a photodiode 210, a transfer transistor 220, a source follower 230, a reset transistor 240, and a selection transistor 250.
A voltage is generated from the photodiode 210 according to the amount of external light received by the photodiode 210. The transfer transistor 220 transfers such a voltage from the photodiode 210 to a gate of the source follower 230. The source follower 230 generates a source-drain current based on the voltage transferred by the transfer transistor 220. The reset transistor 240 applies a reset voltage to the gate of the source follower 230. The selection transistor 250 outputs a voltage generated at the source follower 230 as a pixel output of the unit pixel 200 in response to a signal Sx output from the row driver 40.
The unit pixel 200 performs a reset operation by turning on the reset transistor 240 and outputs a reset voltage according to the reset operation. The photodiode 210 accumulates photo charges according to the amount of light incident after the reset operation. A voltage is generated at the photodiode from the photo charges accumulated for a predetermined period of time, and a resulting pixel voltage is output.
A process of performing analog-to-digital conversion from a difference between the reset voltage and the pixel voltage for a pixel is referred to as CDS (correlated double sampling). FIG. 3 is a block diagram of the image sensor 10 including the conventional ADC (analog-to-digital converter) block 50 with CDS (correlated double sampling). Referring to FIG. 3, the ADC block 50 includes a ramp generator 51, a plurality of comparator circuits 52-1 through 52-N, a gray code generator 53, and a column memory 54.
The ramp generator 51 generates an upward ramp signal Vup and a downward ramp signal Vdn. Each of the plurality of the comparator circuits 52-1 through 52-N includes a pair of switches (e.g., SW11 and SW21), a pair of capacitors (e.g., C11 and C21), and a comparator (e.g., COM1). The gray code generator 53 generates a sequentially increasing gray code. The column memory 54 stores a respective gray code from the gray code generator 53 for each of the comparator circuits 52-1 through 52-N when the respective output of each of the comparators COM1 through COMN makes a logical transition.
The row driver 40 resets pixels (not shown) within a particular row in the pixel array 20. Subsequently, when reset voltages are output from such pixels, switches SW11 through SW1N are turned on (while switches SW21 through SW2N are turned off), and the reset voltages are respectively stored in corresponding capacitors C11 through C1N.
Thereafter, when pixel signal voltages are output from such pixels, the switches SW11 through SW1N are turned off, and the switches SW21 through SW2N are turned on. Thus, the pixel signal voltages are respectively stored in corresponding capacitors C21 through C2N. Here, the ramp voltages Vup and Vdn of the ramp generator 51 are maintained constant such that a difference between respective reset and pixel signal voltages is input across the respective two inputs of each of the comparators COM1 through COMN.
After the reset voltages and the pixel signal voltages are stored in the capacitors C11 through C1N and C21 through C2N, the ramp voltages Vup and Vdn from the ramp generator 51 start increasing and decreasing, respectively, in opposite directions. With such change of the ramp voltages Vup and Vdn, the respective inputs of each of the comparators COM1 through COMN eventually becomes equal and then reverses at a respective time point when a respective output makes a logical transition.
Such a respective time point for each of the comparators COM1 through COMN depends on the respective pixel signal voltage. Thus, such a respective time point indicates the level of the respective pixel signal voltage as represented by the respective gray code from the gray code generator 53 at that time point. The column driver 60 sequentially outputs such gray code values stored in the column memory 54 in response to a control signal received from the control circuit 30.
Unfortunately, the image sensor 10 uses the ramp signal generator 51 that is separately installed within the image sensor 10. Thus, a large circuit area is required in the prior art. In addition, the ramp signals generated by the ramp signal generator 51 may have poor monotonicity such that analog-to-digital conversion cannot be performed accurately in the image sensor 10 of the prior art.